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Bar stock, also (colloquially) known asblank,slug orbillet,[1] is a common form of raw purifiedmetal, used by industry tomanufacture metal parts and products. Bar stock is available in a variety ofextrusion shapes and lengths. The most common shapes are round (circular cross-section), rectangular, square andhexagonal. A bar is characterised by an "enclosed invariant convex cross-section", meaning thatpipes, angle stock and objects with varying diameter are not considered bar stock.
Bar stock is commonlyprocessed by a sequence ofsawing,turning,milling,drilling andgrinding to produce a final product, often vastly different from the original stock. In some cases, the process is partially automated by specialized equipment which feeds the stock into the appropriate processing machine.
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Most metal produced by asteel mill or aluminium plant is formed (viarolling orextrusion) into long continuous strips of various size and shape. These strips are cut at regular intervals and allowed to cool, each segment becoming a piece of bar stock. A good analogy ispasta-making, in which lumps of dough are extruded into variouscross-sectional shapes; cut into lengths; and then dried in that form. The cross-sectional shapes of pasta vary from simple bar or tube shapes (such aslinguine orpenne) to more elaborate extrusions (such asrotelle,fiori, orrotini). The same is true of metal bar stock. The most common shapes areround bar (also calledrod),rectangular bar (includingsquare bar, the special case of equal sides), and hexagonal bar (usually calledhex bar for short).Tube andpipe are similar, but have hollow centers and are traditionally not called "bar" in industrial usage. (However, a product called hollow bar, essentially tube but with custom-orderable OD and ID and thus custom wall thickness, is marketed for lathe bar work which can benefit from obviation of drilling and rough boring.) Also similar in concept, but not called "bar", are thecommon structural shapes such as angle stock and channel stock. These are commonly available in steel and aluminum; the names "angle iron" and "channel iron" are still commonly used (informally) even though their literal namesake,wrought iron, has been replaced by steel and aluminum for most uses.
In amachine shop, bar stock andplate are often calledbillet, although in arolling mill, that word refers to a piece of metal that hasnot yet been rolled into bar.
A machine shop typically has a storage area containing a large variety of bar stock. To create a metal component, a bar of sufficient volume is selected from storage and brought to the machining area. This piece may then besawed,milled,drilled,turned, orground to remove material and create the final shape. In turning, for large-diameter work (typically more than 100 millimetres (3.9 in), although there is no universal threshold), a piece of the bar is cut off using a horizontal bandsaw to create ablank for each part. The blanks are then fed into a chucking lathe (chucker) whichchucks each one in turn. For smaller-diameter work, the entire length of bar stock is more often fed through the spindle of the lathe. The entire bar rotates with the spindle during the part-machining cycle. When the cycle ends and one part is done, the chuck opens, the bar is pulled or pushed forward ("fed") by any of various automatic means, the chuck closes, and the next cycle begins. The last step of the cycle is to cut off the machined part from the bar, which is called "parting it off" and is achieved with a "cutoff" or "part-off" tool, atool bit that grooves the bar all the way down to the centreline, causing the part to fall off. Then the cycle repeats.
The not-yet-cut bar protruding from the back of the spindle, rotating quickly, can present a safety hazard if it is sticking out too far and unconstrained from bending. Thus sometimes long bars must be sawn into shorter bars before being fed as "bar work" (which is the term for such work).
CNC lathes and screw machines have accessories called "bar feeders", which hold, guide, and feed the bar as commanded by the CNC control. More advanced machines may have a "bar loader" which holds multiple bars and feeds them one at a time into the bar feeder. Bar loaders are like magazines for part blanks (or pallets for milling work) in that they allow lights-out machining. The bar loader is filled with bars (or the magazine or pallet with part blanks) during working hours, and then it runs during the night unattended. Given that there is no human around to detect if something went wrong and the machine should stop, there are various kinds of sensors that are used to detect this, such as load meters, infrared beams, and, in recent years,webcams, which are placed inside the machine tool's enclosure and allow remote viewing of the cutting action.
Bar stock is widely used in many industries and can be seen in many different industrial processes. These processes includeforging,extrusion,machining, and many more. In forging, billets are heated to high temperatures before a press pushes theworkpiece into the shape on the die. These presses operate at very high forces to make the desired changes to the product. Extrusion uses rollers that push the heated bar stock through a set of dies which will determine the shape of the workpiece. Machining is a subtractive process that utilizes bar stock and various cutters and tools to make intricate details that are not possible through other processes.[2]
Tostock every possible size of bar stock (every possible fraction of a millimeter or inch in diameter or thickness) is impossible. Thus, bar stock is stocked by metals supply houses in various standard sizes, arrayed in discrete steps. For example, round bar with diameters of even millimeters (or in the US, on the eighths of an inch) can usually be ordered from standingstock. Bar diameters of nonstandard sizes can also be obtained, but only as a separate mill run from therolling mill. Thus they are much more expensive than the standard sizes, can take much longer delivery time, and are not desirable as inventory for the supply house or the machine shop (because the chance of selling or using any particular custom size is slim).
Sometimes it is necessary that the bar not be very much larger than the intended part, because themetallurgical properties of some metalalloys in some finishing processes may vary by how far inside the bar the metal lies. Thus anengineering drawing will specify a certain size (or a maximum size) that the bar may start out as. These specs face the aforementioned limitation of stocking sizes versus custom mill runs; standard sizes are used wherever possible to avoid wasted expense and needless delays.
A drill rod istool steel round stockground to a tighttolerance diameter; it is usually ± 0.0005 in (0.0127 mm). In the UK the name "silver steel" is oftensynonymous and sometimeshyponymous. Its origin was in reference to the shiny ground appearance (not to anysilver alloying content). Drill rod diameters range from 0.0135 to 1.5 in (0.34 to 38.10 mm); in the United States diameters smaller than27⁄64th of an inch (11 mm) are made inletter drill sizes andnumber drill sizes, in addition to fractional sizes. Lengths are usually one or three feet (0.3048 or 0.9144 m). It is commonly used to makedrill bits,taps,reamers,punches,dowel pins, andshafts.[3] Note that the numbered sizes are different from the drill numbered sizes starting at 52. These sizes are:[citation needed]
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Drill blanks have an undersize tolerance of +0/−0.0002 in (0.00508 mm), while reamer blanks have an oversize tolerance of −0/+00.0002 in (0.00508 mm).
Some mills also sell square stock that is held to the same tolerances under the name "drill rod".[3]
Commonly available material grades in the U.S. are A2, D2, M2, M42, O1, S7, W1, andhigh speed steel (including M2/M7).[4]
Ground flat stock isannealed steel that has beenground to close tolerances (compare todrill rod). There are four types of materials available:O-1 tool steel,A-2 tool steel,A-6 tool steel, and1018 steel (low-carbon or low-carb steel). Lengths are either 18 or 36 in (457 or 914 mm) long, various widths up to 16 in (406 mm) are available, and thicknesses range from1⁄64 to 2.875 in (0.40 to 73.03 mm).[5][6][7]
Some geometrical sizes are known asgauge plate.[8]